The present invention relates in general to optical filters, and in particular to a fiber grating package for reducing wavelength variations in optical gratings.
Fiber Bragg gratings (hereinafter referred to also as xe2x80x9cfiber gratingsxe2x80x9d or simply xe2x80x9cgratingsxe2x80x9d) are well known and widely used in a variety of optical applications. In general, a fiber grating is formed by providing a periodic variation in the refractive index of the core of an optical fiber. The periodic variations or gratings in the fiber core cause reflection of a particular Bragg wavelength given by xcexB=2nxcex9, where n is the mean refractive index of the grating, and xcex9 is the grating period. All other incident wavelengths are transmitted through the grating.
Thus, it is well known that by choice of n and xcex9, a fiber grating may be effectively utilized as an optical filter for filtering a desired wavelength from an optical signal. In fact, due to their narrow pass band and relatively inexpensive cost to produce, fiber gratings have developed as key components of many fiber optic communication systems where wavelength or channel selection is critical. Fiber gratings are, for example, widely used for channel selection in wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) communication systems, wherein a plurality of distinct optical wavelengths or channels are multiplexed and propagated over an optical medium to a plurality of receivers. In these systems, the channels or wavelengths chosen for transmission, as well as the channel spacings, are selected to correspond to an International Telecommunication Union (ITU) channel grid, wherein channel spacing may be, for example, 50 or 100 GHz. Reliable selection of a particular ITU channel from a WDM signal is essential to proper functionality of a WDM system.
One difficulty associated with the use of fiber gratings for channel selection relates to variations of resulting from axial strain, i.e. compression or tension, on the fiber including the grating. It is also known that n and xcex9 is temperature-dependent. Variations of n and xcex9 result in corresponding variations in the Bragg wavelength, and thus can significantly effect wavelength selection in a system incorporating a fiber grating.
Several fiber grating compensation schemes have been proposed and attempted for minimizing the effects of axial strain and/or temperature variation on the Bragg wavelength of a grating. To date, however, each of the known approaches to providing compensation for axial strain and/or temperature variations have failed to provide a sufficiently reliable and cost-effective device.
Accordingly, there is a need in the art for a fiber grating package that efficiently and reliably obviates the effects of axial strain the Bragg wavelength of a fiber grating. There is also a need in the art for a fiber grating package that provides reliable compensation for temperature dependency of the Bragg wavelength. There is a further need in the art for art for a fiber grating package, which may be efficiently and cost-effectively produced, minimized power consumption, and is of minimized size.
The present invention is organized about the concept of providing a fiber grating package that substantially reduces Bragg wavelength variations resulting from axial strain on a fiber. The package includes a fiber segment including a Bragg grating. The segment is fixed between two points separated by a distance less than the length of the segment. As a result, the grating is bent to allow bending or rotation of the segment in response to changes in the relative positions of the ends of the fiber segment. In one embodiment, the package may include a temperature control structure and control electronics for heating the grating to a stable desired temperature using temperature feedback. Electronic control can be internal or external to the package depending size requirements for the package.
In particular, a fiber grating package consistent with the invention includes a housing with a segment of optical fiber having a Bragg grating region fixed thereto. The fiber segment has a first end fixed in a first location on the housing and a second end fixed in a second location on the housing. The shortest distance between the first location and the second location is less than the length of the segment, the segment thereby being bent into an arcuate shape between the first and second locations. Fixing of the segment at both ends of the housing isolates the segment from external axial strain. The bending of the segment between the first and second locations allows for rotation or bending in response to relative movement between the first and second locations. Axial strain on the fiber is, therefore, significantly reduced.
The first and second ends may be fixed to the housing by respective glass capillaries. The capillaries may be disposed in respective strain relief assemblies, which are fixed to the housing. In one embodiment, the housing includes top and bottom housing portions. The capillaries or the strain relief assemblies may be disposed in respective slots in the top and bottom housing portions. The housing may include a slot therein in which the segment is disposed. The slot may be formed in the top housing portion, and may be in a j-shape.
According to another aspect of the invention, the package may include a temperature control structure disposed at least partially in the housing. The temperature control structure may have a resistive heating trace disposed thereon, and a heat spreader may be thermally coupled to the resistive heating trace. The Bragg grating region may be disposed adjacent the heat spreader to allow heating of the Bragg grating region to a desired temperature. The fiber segment may be disposed in a slot formed in the heat spreader, e.g. an s-slot.
The temperature control structure may extend outward from the housing to allow connection of control electronics for controlling current flow through the heating trace. The control electronics may be configured to control the current flow through the heating trace based on a resistance across a thermistor disposed on the temperature control structure adjacent the Bragg grating region. The heat spreader and a portion of the temperature control structure including the resistive heating trace may be disposed in an associated pocket in the bottom housing portion. The control electronics may, however, be disposed on the temperature control structure, and the temperature control structure may be disposed entirely within the housing.